The system of 55 Cancri has five detected planets that span a broad range of masses, from about 10 times Earth to 4 or 5 times Jupiter. All evidently travel on more or less circular orbits, like planets in the Solar System. The host is a Sun-like G8 star located at a distance of 12.53 parsecs (41 light years) in the constellation Cancer. It is cooler and less massive than our Sun, with a bolometric luminosity only 60% Solar (Fischer et al. 2008).

The yellowish primary star has a small binary companion of spectral type M4 and mass 0.26 MSOL, orbiting at a semimajor axis of about 1000 AU (Desidera & Barbieri 2006). The wide separation between the two stars suggests that neither would substantially inhibit the evolution of planets around the other. In fact, the red dwarf companion, 55 Cancri B, is a potential exoplanet host in its own right. Future radial velocity searches will establish whether it harbors its own planetary system.

Takeda and colleagues note that the mass and age of 55 Cancri are poorly constrained. This uncertainty seems to be a function of the star’s unusually high metallicity (0.315). Nevertheless, they are confident in assigning values very close to Solar for both parameters: a mass of 0.96 MSOL and an age of 5 billion years (Takeda et al. 2007). This estimate is supported by the star's leisurely rotation period of 39 days (Fischer et al. 2008). Thus the star's planetary companions may have reached an evolutionary stage similar to those in our own system, whose age is estimated at 4.6 billion years.

system architecture

Like the Solar System, the system of 55 Cancri has clearly demarcated inner and outer regions (see comparative diagram). The inner system comprises a cluster of four planets, all orbiting within a semimajor axis of 0.8 AU. This configuration is dominated by its most massive member, the Jupiter-size second planet (b), and it includes a Super Earth of about 10 MEA (e) and two additional objects that fall between the ice giants and the gas giants in mass (c, f). Then, after a gap of 5 AU, the outlying fifth planet (d) traces a wide orbit with a period of more than 14 Earth years, even longer than Jupiter's period of 12 years. This fifth planet is 55 Cancri's true Jupiter analog. Its minimum mass of almost 4 MJUP and its relatively circular orbit must have exercised strong constraints on the system’s evolution, just as Jupiter did in our system.

The resulting configuration fulfills key predictions regarding the correlation of metallicity with planetary evolution (Greaves et al. 2007). Enhanced stellar metallicity is associated with giant planet formation as well as inward migration. In the case of 55 Cancri, four giant planets evolved, with three of them (planets b, c, and f) orbiting close to the central star and the fourth and most massive (d) remaining beyond the system's ice line.

The three innermost planets bear a strong family resemblance to the system of Gliese 876. Each system contains a star-hugging ensemble of planets comprising two gas giants (or quasi-gas giants) orbiting in or near a mean motion resonance, plus a smaller planet on an interior orbit. In each case, the inner planet is a tidally circularized Super Earth that has evidently been shepherded into its present position by the inward migration of the two more massive planets (Ida & Lin 2005, Fogg & Nelson 2005, Mandell et al. 2007; see also Crowded Orbits).

55 Cancri resembles Gliese 876 in another important way, insofar as in both systems the orbital inclination of the most massive planet has been estimated by photometric observations (Rivera et al. 2005, Fischer et al. 2008). If we make the reasonable assumption that the remaining planets in each system have similar inclinations, then we can calculate the actual mass as well as the minimum mass for each one. In the case of 55 Cancri, the inclination is approximately 53 degrees.

five planets

1. The inner planet, designated 55 Cancri e, has a minimum mass of about 7.66 MEA (actual mass about 9.5 MEA), a semimajor axis of 0.038 AU, and a period of less than three days. Unlike most known extrasolar planets, this one may be primarily rocky rather than gaseous or icy. Otherwise, over the 5-billion-year lifetime of 55 Cancri, its tight orbit would likely result in total mass loss through photoevaporation (Lecavelier des Etangs 2006). As a rocky world subject to intense heating and magnetic flux from the nearby host star, this first planet of 55 Cancri may be torn by constant volcanic activity.
   
   
2. The second planet, 55 Cancri b, has a semimajor axis of 0.11 AU (just outside the nominal range for Hot Jupiters) and a minimum mass of 0.84 MJUP (actual mass about 1.05 MJUP). Its atmosphere must be too hot to permit substantial cloud formation (Sudarsky et al. 2003). The planet is plausibly interpreted as a smooth, deep blue sphere. Similar in mass and diameter to Jupiter, this planet has dominated the evolution of the inner system. Its period of 14.65 days places it very close to a 3:1 mean motion resonance with its outer companion.
   
   
3. The third planet, designated 55 Cancri c, is a smaller, sub-Saturn world of about 0.17 MJUP (actual mass 0.21 MJUP = 67 MEA) with a semimajor axis of 0.24 AU. Its period of 44.4 days is almost triple that of its inner neighbor. The third planet is also likely to be a clear blue world, possibly similar in appearance to Neptune. Its mass places it near the uncertain boundary between ice giants and gas giants.
   
   
4. The fourth planet, designated 55 Cancri f, is smaller still, with a minimum mass of 0.14 MJUP (actual mass 0.18 MJUP = 57 MEA) and a semimajor axis of 0.78 AU. It also straddles the boundary between ice giants and gas giants, as it is far heavier than a confirmed ice planet like Neptune or GJ 436 b but lighter than a confirmed gas planet like Saturn. Since its primary star is cooler than our Sun, planet f occupies the system's classical habitable zone, despite an orbital radius similar to Venus. This planet is therefore likely to be a "water giant," with extensive white clouds of water vapor or ice crystals. Given its separation from the primary, it probably sustains a fairly rapid rotation, which would create dynamic weather patterns throughout its dense hydrogen-helium atmosphere.
   
   Planet f may be far enough from its host star to support a family of moons (see also Potential Exomoons). Given the approximate mass scaling of 1:10,000 proposed by Canup & Ward (2006), any co-formed moons of this planet would be quite small, of Lunar mass or less. However, we cannot yet rule out the possibility that planet f has captured a rocky moon whose mass follows the 1:100 scale that characterizes the Earth-Luna system. In such a scenario, the hypothetical captured moon might be even more massive than Mars. Given our present state of knowledge, however, this outcome seems unlikely.
   
   
5. With a semimajor axis wider than Jupiter's, the fifth planet of 55 Cancri could be accurately characterized only after the physical and orbital elements of the inner planets were resolved (Fischer et al. 2008). Its minimum mass is now estimated at 3.92 MJUP (actual mass 4.9 MJUP); its semimajor axis is 5.84 AU, with an orbital eccentricity of 0.06; and its period is almost 15 years.

   55 Cancri d seems more likely than most known extrasolar planets to resemble Jupiter and Saturn, especially because it may possess colorful cloud bands and even a system of rings. Unlike the system's four inner planets, the fifth planet is also likely to maintain a super-Jovian family of moons, with satellites potentially as massive as Mars or Venus. Unfortunately, such moons are likely to be cold and barren unless gravitational stresses induce substantial heating through volcanism.

migration and evolution

The fifth planet has probably undergone little or no inward migration during its evolution, in contrast to the giant planets in the inner system. These planets evidently spiraled inward through the system's primordial gas disk to reach their present positions, in a process known as "Type II migration" (see Evolution of Planetary Systems). Gravitational perturbations forced rocky planetesimals into the shrinking area within the radius of the second planet’s orbit, where the planetesimals underwent collisions, accretions, and ejections. The result was the formation of the system's closest planet, the hot Super Earth that we now observe in its star-grazing orbit of less than three days (Fogg & Nelson 2005, Raymond et al. 2006b, Mandell et al. 2007).

Last update July 2008